The present invention relates to a three-phase reluctance motor comprising a rotor having a plurality of convex poles.
A three-phase motor particularly of a variable reluctance type as disclosed in Japanese Patent Application KOKAI Publication No. 8-116651 have been conventionally known. This motor includes a stator having six poles and a rotor made of a soft magnetic material and having four convex poles. Coils of phases A, B, C, A, B, and C are concentrically wound about the six poles located at an equal pitch along the circumferential direction of the stator, respectively. This motor is constructed in a structure in which each pair of magnetic poles shifted by 180.degree. from each other and opposing to each other with the rotor 2 interposed therebetween are of one same phase and two coils of each pair of magnetic poles form a coil of one phase by serially connecting the coils with each other.
In the structure as described above, four of six magnetic poles are selectively excited by applying a current to two of three phase phases, and the four magnetic poles thus excited are shifted along the circumferential direction at predetermined timings of switching the current to attract teeth of the rotor and rotate the rotor in a predetermined direction.
This reluctance type motor is a motor using a principle that the rotation torque acts in such a direction in which the reluctance (or magnetic resistance) between the stator and the rotor is minimized. The motor also adopts another principle that the torque generated becomes larger as the change of the reluctance per unit rotation angle is larger and as the magnetomotive force generated by the coil current between the stator and the rotor is greater. Therefore, a conventional reluctance type motor is arranged such that the width of each magnetic pole of the stator and the width of each convex pole are substantially equal to each other and each arc angle between the convex poles is considerably small as disclosed in U.S. Pat. No. 5,459,385.
However, the reluctance type motor of the structure as described above has a drawback that braking torque is generated in the direction opposite to the rotational direction of the rotor due to a transient phenomenon when switching the current or leakage of magnetic fluxes to magnetic poles not excited and leads to reduction of the efficiency and the output, thereby hindering high speed rotation. Therefore, reluctance type motors are not positively used at present.